This invention relates to failure diagnosis devices for automotive engines, and more particularly to failure diagnosis devices which are capable of judging whether a proper condition for effecting failure diagnosis has been reached.
Generally, failure diagnosis devices for automotive engines (including the control devices for engines having a failure diagnosis function) effect the failure diagnosis of the control system of an automobile as follows. First, the operation states of the engine are detected after the water temperature, the rpm, and the air intake amount, etc., are within predetermined ranges. Then, the detected values are compared with preset operation states of the engine. If they are outside of predetermined ranges from the predetermined operation states, the system is judged to be in a failure, and the codes that are set within the failure diagnosis device are stored in memory and are displayed on a monitor display.
For example, a failure diagnosis of the exhaust gas recirculation (EGR) system is effected as follows. The outputs of EGR temperature sensor, etc., are detected when the rpm, the water temperature, the air intake amount, etc., of the engine are within predetermined ranges and hence the exhaust gas recirculation system is in an operable state. If the outputs of EGR temperature sensor, etc., are outside of normal ranges, the exhaust gas recirculation system is judged to be in failure. The failure codes set at the failure detection are stored in memory and displayed.
The above conventional failure diagnosis device has the following disadvantage.
The automobile must be driven for a certain mileage or time before an engine condition is reached under which the failure diagnosis can be effected properly. Thus, if the failure status is examined after the failure code has been cleared from the memory, a wrong judgment that the system is not in failure may be made by the failure diagnosis device, the judgment of which is based on the failure codes stored in the memory.